In recent years, a so-called air bag device is in wide use as a safeguard provided in front of an automobile seat. The air bag device is made up of an air bag in a sack state, a sensor perceiving a shock which is administered to an automobile, and an inflator in which a gas is introduced into the air bag by action of the sensor to blow it up in a moment. The air bag is normally folded and contained, for example, within the steering wheel. When an automobile receives a strong shock, for example, by a crash, the sensor located in the center of the dashboard floor or front floor perceives the shock which allows reaction of a gas-generating agent such as sodium azide contained as a main component in the inflator to evolve a nitrogen gas that is sent to the air bag to inflate it in a moment. That is, the air bag device perceives a shock on occurrence of a motor vehicle accident to inflate the air bag in a moment and the inflated air bag effectively mitigates the impact on riders, thus playing a major role in protecting the riders from the accident. In general, the base cloth for air bags of such air bag devices is woven cloth formed of synthetic resins such as nylon resin, on one side of which (the side forming the inside of the air bag) chloroprene rubber films (JP-A-49-55028; The term "JP-A" as used herein means an "unexamined published Japanese patent application) or silicone rubber films (JP-A-2-270654) are formed, and the air bags are prepared by sewing in a sack state several sheets of such base cloth which are cut to specified shapes, respectively. The films formed on the inside of the air bags present gas tightness to the woven cloth, and in addition, prevent the nylon woven cloth from being directly exposed to a gas of high temperatures which is introduced into the air bags in a burst once the air bags are unfolded to prevent the nylon resin from fusion and deterioration, which in turn prevent riders from exposure to the gas of high temperatures. Therefore, some resistance to heat is required for the films. On the other hand, as air bags must be normally folded and contained, for example, within the steering wheel, the air bags which can be folded as compactly as possible are desirable in view of space saving for containment. From this viewpoint, chloroprene rubber was mainly used for the above-mentioned films at first. However, the chloroprene rubber has the disadvantages of insufficient thermal resistance and insufficient durability which lead to shortened life of the air bag. In addition, to prevent the danger of fire and blast which may break out on crashing, it is necessary for air bags to have fire retardance. The chloroprene rubber is so insufficient in fire retardance that fire retardants made of silicone must be further applied to regions of chloroprene rubber films which are exposed to the blast.
On the other hand, in air bags coated with silicone rubber, it is possible to give fire retardance to coated films themselves by incorporating known fire retardants into the silicone rubber, thus to exclude an additional step of applying the fire retardants to base cloth. For this reason, considerable attention has been directed to silicone rubber coating agents providing more excellent resistance to heat and weather than the chloroprene rubber.
The air bags coated with the silicone rubber also are usually contained in the steering wheel and blown up by the blast on crashing. Then the coated films also instantly stretch corresponding to stretching of the base cloth, and therefore, mechanical strength and stretching properties are required for the silicone rubber films. Consequently, base polymers of high polymerization degrees and high viscosities are utilized and further blended with reinforcing agents, fire retardants, adhesives, and the like. However, it is difficult that these silicone compositions which are highly viscous in general are applied to base cloth in desired amounts of 30 to 100 g/m.sup.2 by a coating method such as knife coating. Accordingly,.the compositions need a stage of diluting with organic solvents such as toluene or xylene to be adjusted to viscosities so that coating such as knife coating is readily carried out and curing coated films, while evaporating the solvents in a drier.
Therefore, it is difficult, in general, to apply the rubber for air bags to base cloth in usually required amounts of 30 to 100 g/m.sup.2 by a coating method such as knife, roll, or gravure coating and it is commonly performed that the rubber is diluted with organic solvents to be adjusted to viscosities suitable for coating prior to coating.
However, these organic solvents have the disadvantages of possibly catching fire due to static electricity, health hazard caused by inhalation or touch to skin, and in addition, a heavy cost is required to recover evaporated solvents. The organic solvents further introduce the problem of air pollution when they are not recovered. Use of the organic solvents is coming to be regulated in all areas in recent years.
To attain low viscosities suitable for applying coating compositions to base cloth through knife coating or the like without using the organic solvents, there is a method of lowering polymerization degrees of base polymers of the coating compositions. However, such compositions fail to form cured films with sufficient mechanical strength when applied to the base cloth. When air bags are unfolded, the films may craze by blast of high temperatures, failing to sufficiently blow up the air bags because of gas leaks.
In addition, conventional rubber coating compositions have the disadvantage of forming cured films with great tackiness to cause blocking of the films to one another.
On the other hand, a variety of silicone aqueous emulsion compositions which form elastomeric substances after removal of water have been proposed for film-formable emulsion type silicone compositions. Examples of such compositions proposed include those composed of a polydiorganosiloxane blocked by hydroxyl groups at both the ends of the molecular chain, a polyorganohydrogensiloxane, a polyalkyl silicate, and a tin salt of fatty acid as described in JP-B-38-860 (The term "JP-B" as used herein means an "examined Japanese patent publication"); those composed of a polydiorganosiloxane blocked by hydroxyl groups at both ends of the molecular chain, a silane containing three or more functional groups, and a tin salt of fatty acid as described in JP-B-57-57063; those composed of a polydiorganosiloxane, a polyorganohydrogensiloxane, and a platinum compound as described in JP-B-58-17226; and emulsion polymerization products from a cyclic organosiloxane and a functional group-bonded organoalkoxysilane as described in JP-A-54-131661.
However, although elastomeric substances formed of these emulsion compositions are excellent in thermal resistance, water repellency, weathering resistance, and transparency, they are inferior in mechanical strength, which indicates that they are unsuitable, for example, as coating agents.
Some means of adding colloidal silica as a reinforcing agent have been proposed to improve the mechanical strength. One of such means utilizes hydrosilylation. For example, it is described in JP-A-54-52160 to add colloidal silica to an emulsion composed of a polydiorganosiloxane containing a vinyl group in the ends of the molecular chain or in side chains, a polyorganohydrogensiloxane, and a platinum compound (catalyst). In addition, JP-A-56-36546 discloses a process for adding colloidal silica in which an emulsion composed of a polydiorganosiloxane blocked by vinyl groups at both the ends of the molecular chain, a polyorganohydrogensiloxane, and a platinum compound (catalyst) is heated to form a crosslinking structure and colloidal silica is then added to the emulsion.
However, elastomeric substances formed by removal of water from these emulsion compositions have only insufficient bonding, namely interfacial adhesion, between colloidal silica and the polyorganosiloxanes and uniform dispersability of colloidal silica also is poor, failing to sufficiently offer the reinforcing effect of silica on silicones.
To resolve the problem described above, as crosslinking processes different from those described above, JP-A-61-16929 and JP-A-61-271352 disclose processes for preparing emulsions in which, for example, a diorg-anosiloxane of a low polymerization degree which is blocked by hydroxyl groups at both the ends of the molecule and an alkoxysilane containing three or more functional groups undergo emulsion polymerization in the presence of an acidic colloidal silica. However, it is difficult to contain a starting siloxane and the colloidal silica in the same micelle in initial homogenizing, and in the emulsion thus formed, unreacted siloxane and silica coexist with condensation products between them in the micelle, failing to achieve improvement in the mechanical strength.